1912 (3 page)

James Weddell championed the idea of sailing directly to the bottom of the world, but it was the search for a different pole that had piqued the interest of most scientists and the public during the seventeenth and eighteenth centuries. Although the geographic poles mark the location on the surface of the Earth around which the axis of our planet rotates, there are many others. There is a pole for the greatest distance from a coast (the Pole of Inaccessibility), one for the most frigid place (Cold) and even one for the spot with the greatest range in atmospheric pressure (Variability). In Weddell's time it was the magnetic version that fascinated. Spurred on by the British Royal Navy's desire to understand how the world's compasses might be better used, science gained equal footing with exploration, and became less dependent on enthusiastic amateurs and haughty employers.

The eleventh-century Chinese discovery that the mineral lodestone would naturally point north–south if freely suspended had led to the development of compasses that enabled navigators to plan and explore routes around the world with increased
confidence and safety. But navigating by compass was not foolproof. Over time compasses subtly changed the direction in which they pointed; and the further you went polewards, the more erratic they seemed to become. For a country such as Britain, dependent on ships for trade and military muscle, the situation was serious: a drifting pole could become a hazard for ships.

New data was needed, to test the scientific community's understanding of the planet's magnetism and exploit it. More accurate hydrographic surveys and maps showing magnetic field irregularities would improve the accuracy of navigation, reducing passage times and preventing disaster—and helping in sovereignty claims. The pressure for this data only increased with the shift from wooden craft to metal shipping, further distorting the magnetic signal.

The first to raise the issue was Robert Norman, who in 1581 published a book called
The New Attractive
after he became frustrated at the way compass needles would incline below the ‘plaine of the horizon': no matter how carefully he prepared his needles in London, once they were magnetised the north-facing part would dip without fail. Norman found he had to snip the end off the north-seeking part of his needles, thereby allowing them to balance on the pivot. He went on to measure this effect by setting up a magnetised needle vertically and reading the angle of magnetic dip. His dipping needle showed they always pointed to 72°. Norman felt it was something inherent to the needles themselves.

In 1600 Queen Elizabeth I's physician and scientist, William Gilbert, proposed a different, revolutionary idea. It was not the needles themselves that caused the dip, Gilbert argued. Instead, the phenomenon could best be explained if the planet had something akin to a powerful bar magnet inside it. Gilbert did not understand the cause, but we now know the magnetic field is produced by a solid inner iron core surrounded by fluid iron. It
is this outer part that acts like a spinning conductor in a bicycle dynamo. Rather than frantically peddling, though, the Earth's system is run by heat from the decay of radioactive elements left over from our planet's formation. The resulting swirling molten iron in the outer core is electrically charged, creating a continuously changing electromagnetic field.

The upshot of all this is that a freely hanging magnetised needle will align itself to the line of magnetic force. Scatter iron filings on a sheet of paper covering a bar magnet and the filings will rapidly align themselves to the magnetic field, tracing a semicircle of iron around the bar, connecting the poles at either end. Depending where on the Earth's surface you stand, the strength and direction of the horizontal and vertical parts of the magnetic field will vary. In the tropics the horizontal force dominates the magnetic field, so a needle will tend to sit parallel to the surface. But, as Robert Norman found, approaching polar regions the amount of dip increases as the field sweeps around the Earth and returns to the magnetic poles. As a result, a needle in their vicinity will approach a more vertical position. Norman's dipping compass enabled people to measure the vertical part of the field, and it could also be an asset for navigation, providing a measure of latitude and, ultimately, proximity to a magnetic pole.

The magnetic core is tilted at a slight angle off the axis of our planet's rotation, by some 11°. However, although a bar magnet in the Earth is a great concept, it is only an approximation of what is going on under our feet. Swirling molten currents, the magnetism of surrounding rocks and changes in the sun's activity all complicate this notion of a simple magnetic field. The result is one of the more perplexing concepts in Earth science: the presence of two different types of magnetic poles in each hemisphere. The better-known magnetic poles are where the field dips at right angles to the surface, while the
geomagnetic poles are the theoretical locations for the axis of the Earth's magnetic field if it did truly work like a bar magnet, as William Gilbert envisaged. In each hemisphere these poles are more than a thousand kilometres apart, and over time change their absolute and relative positions to one another as they move across the surface.

Over the next century it was realised that the lines of equal magnetic force were not evenly distributed around the world. An alliance between the Royal Navy and the Royal Society sought to solve this conundrum. In 1693 grandiose plans were made for a purpose-built vessel,
HMS Paramore
, to investigate these magnetic variations on the world's first scientific voyage. Its leader was to be a civilian, the polymath Edmund Halley, remembered today mainly for the comet whose path he predicted. Since his schooldays Halley had been interested in the Earth's magnetism and, having gathered together the limited observations made around the known world, he proposed that the magnetic field was not stable but in fact slowly drifting westwards. To account for these observations, Halley imagined an Earth made up of four magnetic poles, with two in each hemisphere. The
Paramore
would test these ideas by direct measurement. If he had time, Halley would also look for the coastline of
Terra Australis Incognita
.

Halley left Britain in October 1698, only to have a spectacular falling out with an officer on board who repeatedly criticised his handling of the ship in front of the crew. The
Paramore
ignominiously returned to Britain in June 1699, its mission barely started. Halley set out again, this time without the surly officer, and the expedition reached the edge of the Antarctic region at 52°S in the South Atlantic on 1 February 1700. Halley did not see land but spotted his first ‘islands of ice'. Thinking at first it was ‘land with chaulky cliffs and the topp all covered with snow', he realised his mistake when the
Paramore
managed to heave to and he
discovered there was a real risk of becoming trapped among the bergs. Halley beat a hasty retreat north. On his return to Britain, he published his measurements of the magnetic field, and showed that in the North Atlantic compasses did not point true north. The geographic and magnetic poles were not one and the same.

Halley never got close to either of the magnetic poles. If he had, he would have quickly become aware of huge swings, often daily, in their locations. This was first remarked upon by the London scientific watch and compass maker George Graham in the early eighteenth century. Graham took more than a thousand observations in 1722, reporting to the Royal Society that he had found a rhythm in the number of times his compass needles swung back and forth each day, with the greatest change taking place between noon and four in the afternoon.

Halley's and Graham's results showed there was considerably more to the Earth's magnetism than first thought, and that the problem could not be solved piecemeal. By the nineteenth century, scientists set out to make a more systematic study by undertaking simultaneous measurements of the Earth's magnetic properties. At one of those rare moments when European nations were not fighting, the German scientist and mathematician Carl Friedrich Gauss persuaded observatories across Europe and Asia to collaborate in his Magnetic Union. By 1834 there were twenty-three stations measuring data on the strength of the magnetic field, the direction of compass readings and other parameters, all using Gauss's instruments, which allowed the results to be directly compared. The subsequent Magnetic Crusade established a network of British-run stations around the world. Soon an array of global observers was taking synchronised measurements.

To help make sense of the observations being patiently collected and to test mathematical models of the planet's magnetic field, it was critical that the surface locations of the
magnetic poles were known. In 1831 the naval officer James Clark Ross led the British effort north by sledging out from his ice-bound ship,
HMS Victory
; after several weeks travelling with Inuit companions he succeeded in finding his compass needle sitting upright. He had reached the North Magnetic Pole. Impressed by Ross's heroics, the Royal Society in 1839 convinced the Navy to send him with two vessels,
HMS Erebus
and
Terror
, to reach its southern counterpart. These forerunners of battleships, designed to bombard targets on land, were heavily reinforced: ideal, reasoned the Royal Navy, for forcing their way through any offending sea ice.

No one knew whether the pole lay on land or sea. After reading of Weddell's travels, Ross favoured the latter.

As Ross headed polewards two other nations were also seeking to reach the South Magnetic Pole and plant their flags on any Antarctic land they might find. A French expedition of two ships set out in 1837, led by the wonderfully named Jules-Sébastien-César Dumont d'Urville, while an American circumnavigation attempt commanded by Charles Wilkes sallied forth in August 1838 with four vessels. Neither made it to the South Magnetic Pole; but by the time Ross reached Hobart he knew of the French and American expeditions. Determined to take another route to the pole, Ross struck out due south from the Tasmanian capital in November 1840, taking with him the young Joseph Hooker as ship's surgeon and part-time scientist. Ross made good progress and, on reaching the pack ice in the early new year, decided to plough the
Erebus
and
Terror
headlong under sail. The experience prematurely aged him—and no doubt many of his crew—yet his luck held. After only four days, Ross's confidence in his ships' ability to survive the pounding
of ice was rewarded when the expedition reached open water.

To find the South Magnetic Pole, Ross had an instrument similar to Norman's dipping compass called a Fox Dip Circle, which allowed the dip and strength of the magnetic field to be measured at sea. Some years earlier the British explorer Matthew Flinders, while surveying the Australian coast, had demonstrated that by ‘swinging' the ship through the thirty points of the compass and taking magnetic measurements at anchor, he could account for any interference from iron aboard the vessel. The ever-present icebergs made this a risky business—and yet, as Ross pushed on, the average readings did indeed show the Fox dipping needle was moving ever closer to the vertical; he was tantalisingly close to the magnetic pole. But, just as it seemed the explorer was to be rewarded for his bravery, Ross met a vista of mountains, preventing further progress. He claimed this new panorama for Britain and named it Victoria Land, after his monarch. Ross calculated the South Magnetic Pole lay eight hundred kilometres southwest inland: somewhere not even the
Erebus
or
Terror
could go.

Hoping he might yet find an ocean route to the magnetic pole, Ross followed the mountainous coastline away to the south. After sixteen days he reached 78°10'S, beating Weddell's furthest south, but found the expedition path blocked again, this time by what Hooker described as ‘a fine volcano spouting fire and smoke'. They had stumbled upon the southernmost active volcano in the world, Mount Erebus, atop Ross Island. To the west was a relatively sheltered anchorage, McMurdo Sound, named after a lieutenant on the
Terror
—but to the east was something unknown to science. Ross wrote that he had discovered ‘a perpendicular cliff of ice between one hundred and fifty and two hundred feet [forty-five and sixty metres] above…the sea, perfectly flat and level on top, and without any fissures or promontories on even its seaward face'. Perhaps
inspired by Matthew Flinders's Great Barrier Reef, Ross named his discovery the Great Ice Barrier. Now known as the Ross Ice Shelf, this new find lived up to its original moniker: the ice cliff extended over 720 kilometres to the east and, save for a small bay filled with whales, offered no ready features or way through. Disappointed, Ross could go no further, and he turned his vessels north for home.